Slurry compositions for making environmental barrier coatings and environmental barrier coatings comprising the same

ABSTRACT

Slurry compositions for making an environmental barrier coating including from about 9 wt % to about 81 wt % water; from about 3 wt % to about 72 wt % primary material; and from about 0.1 wt % to about 18 wt % slurry sintering aid, and environmental barrier coatings including such compositions.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application claims priority to U.S. Provisional Application Ser.No. 61/230,354, filed Jul. 31, 2009, which is herein incorporated byreference in its entirety.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made, at least in part, with a grant from theGovernment of the United States (Contract No. N00019-04-C-0093, from theDepartment of the Navy). The Government may have certain rights to theinvention.

TECHNICAL FIELD

Embodiments described herein generally relate to slurry compositions formaking environmental barrier coatings and environmental barrier coatingscomprising the same. More particularly, embodiments described hereingenerally relate to environmental barrier coatings that are made anddensified using at least one sintering aid.

BACKGROUND OF THE INVENTION

Higher operating temperatures for gas turbine engines are continuouslybeing sought in order to improve their efficiency. However, as operatingtemperatures increase, the high temperature durability of the componentsof the engine must correspondingly increase. Significant advances inhigh temperature capabilities have been achieved through the formulationof iron, nickel, and cobalt-based superalloys. While superalloys havefound wide use for components used throughout gas turbine engines, andespecially in the higher temperature sections, alternativelighter-weight component materials have been proposed.

Ceramic matrix composites (CMCs) are a class of materials that consistof a reinforcing material surrounded by a ceramic matrix phase. Suchmaterials, along with certain monolithic ceramics (i.e. ceramicmaterials without a reinforcing material), are currently being used forhigher temperature applications. These ceramic materials are lightweightcompared to superalloys, yet can still provide strength and durabilityto the component made therefrom. Therefore, such materials are currentlybeing considered for many gas turbine components used in highertemperature sections of gas turbine engines, such as airfoils (e.g.turbines, and vanes), combustors, shrouds and other like components,that would benefit from the lighter-weight and higher temperaturecapability these materials can offer.

CMC and monolithic ceramic components can be coated with environmentalbarrier coatings (EBCs) to protect them from the harsh environment ofhigh temperature engine sections. EBCs can provide a dense, hermeticseal against the corrosive gases in the hot combustion environment,which can rapidly oxidize silicon-containing CMCs and monolithicceramics. Additionally, silicon oxide is not stable in high temperaturesteam, but is converted to volatile (gaseous) silicon hydroxide species.Thus, EBCs can help prevent dimensional changes in the ceramic componentdue to such oxidation and volatilization processes. Unfortunately, therecan be some undesirable issues associated with standard, industrialcoating processes such as plasma spray and vapor deposition (i.e.chemical vapor deposition, CVD, and electron beam physical vapordeposition, EBPVD) currently used to apply EBCs.

Accordingly, there remains a need for environmental barrier coatings toprotect CMCs from the high temperature steam environments present in gasturbine engines.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments herein generally relate to slurry compositions for making anenvironmental barrier coating, the slurry composition comprising: fromabout 9 wt % to about 81 wt % water; from about 3 wt % to about 72 wt %primary material; and from about 0.1 wt % to about 18 wt % slurrysintering aid.

Embodiments herein also generally relate to environmental barriercoatings comprising: at least one transition layer made from a slurrycomposition comprising: from about 9 wt % to about 81 wt % water; fromabout 3 wt % to about 72 wt % primary transition material; and fromabout 0.1 wt % to about 18 wt % slurry sintering aid; and an outer layermade from a slurry composition comprising: from about 9 wt % to about 81wt % water; from about 3 wt % to about 72 wt % primary outer material;and from about 0.1 wt % to about 18 wt % slurry sintering aid.

Embodiments herein also generally relate to environmental barriercoatings comprising: a bond coat layer comprising silicon; a silicalayer; at least one transition layer made from a slurry compositioncomprising: from about 9 wt % to about 81 wt % water; from about 3 wt %to about 72 wt % primary transition material; and from about 0.1 wt % toabout 18 wt % slurry sintering aid; and an outer layer made from aslurry composition comprising: from about 9 wt % to about 81 wt % water;from about 3 wt % to about 72 wt % primary outer material; and fromabout 0.1 wt % to about 18 wt % slurry sintering aid wherein thetransition layer comprises a porosity of from about 0.01% to about 30%by volume of the transition layer, and the outer layer comprises aporosity of from about 0.01% to about 15% by volume of the outer layer.

These and other features, aspects and advantages will become evident tothose skilled in the art from the following disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the invention, it is believed that theembodiments set forth herein will be better understood from thefollowing description in conjunction with the accompanying figures, inwhich like reference numerals identify like elements.

FIG. 1 is a schematic cross sectional view of one embodiment of anenvironmental barrier coating in accordance with the description herein;

FIG. 2 is a SEM cross-section of an EBC coating on a SiC—SiC CMC inaccordance with Example 1;

FIG. 3 is a SEM cross-section of an EBC coating on a SiC—SiC CMC inaccordance with Example 2;

FIG. 4 is a SEM cross-section of an EBC coating on a SiC—SiC CMC inaccordance with Example 3; and

FIG. 5 is a SEM cross-section of an EBC coating on a SiC—SiC CMC inaccordance with Example 4.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments described herein generally relate to slurry compositions formaking environmental barrier coatings and environmental barrier coatingscomprising the same. More particularly, embodiments described hereingenerally relate to environmental barrier coatings that are made anddensified using at least one sintering aid.

More specifically, the EBCs described herein below can comprisesintering aids, which can lower the sintering temperature, therebypromoting the formation of dense EBC layers that can act as a hermeticseal to protect the underlying component from corrosion from the gasesgenerated during high temperature combustion without damaging thecomponent through exposure to high sintering temperatures, as explainedherein below.

The EBCs described herein may be suitable for use in conjunction withCMCs or monolithic ceramics. As used herein, “CMC” refers tosilicon-containing matrix and reinforcing materials. Some examples ofCMCs acceptable for use herein can include, but should not be limitedto, materials having a matrix and reinforcing fibers comprising siliconcarbide, silicon nitride, and mixtures thereof. As used herein,“monolithic ceramics” refers to materials comprising silicon carbide,silicon nitride, and mixtures thereof. CMCs and monolithic ceramics arecollectively referred to herein as “ceramics.”

As used herein, the term “barrier coating(s)” can refer to environmentalbarrier coatings. The barrier coatings herein may be suitable forapplication to “ceramic components,” or simply “components,” found inhigh temperature environments (e.g. operating temperatures of about2500° C.), such as those present in gas turbine engines. Examples ofsuch components can include, for example, combustor components, turbineblades, shrouds, nozzles, heat shields, and vanes.

More specifically, component 10 may comprise EBC 12, which can include abond coat layer 14, an optional silica layer 15 adjacent to the bondcoat layer, at least one transition layer 16 adjacent to bond coat layer14 (or silica layer 15 if present), and an outer layer 18 adjacent totransition layer 16, as shown generally in FIG. 1. Bond coat layer 14may comprise silicon and may generally have a thickness of from about0.1 mils to about 6 mils. Due to the application method as describedherein below, there may be some local regions where the silicon bondcoat is missing, which can be acceptable. For example, in oneembodiment, bond coat layer can cover about 100% of a surface 11 ofcomponent 10, and in another embodiment, about 90% or more of thesurface of the component.

Silica layer 15 can have an initial thickness of from about 0.0 mils toabout 0.2 mils, and the thickness of silica layer 15 can increase overtime. Specifically, in one embodiment, the silicon of bond coat layer 14can oxidize slowly during the service life of the EBC to gradually formsilica layer 15. The oxidation of bond coat 14 can protect theunderlying ceramic component from oxidation, since the bond coat isoxidized rather than the ceramic component. Alternately, as explainedbelow, silica layer 15 can be applied using conventional methods to aninitial thickness as described previously, and can again increase inthickness over time.

Transition layer 16 may include from about 85% to about 100% by volumeof the transition layer of a primary transition material and up to about15% by volume of the transition layer of a secondary material, and inone embodiment from about 85% to about 99% by volume of the transitionlayer of the primary transition material and from about 1% to about 15%by volume of the transition layer of the secondary material.

As used herein, “primary transition material” refers to mullite, BSAS,or a mixture of the two (hereafter “mullite/BSAS mixture”). The“mullite/BSAS mixture,” also referred to as “a mixture of mullite andBSAS,” can comprise from about 1% to about 99% by volume mullite andfrom about 1% to about 99% by volume BSAS, and in another embodiment,from about 70% to about 90% by volume mullite and from about 10% toabout 30% by volume BSAS. As used herein throughout, “secondarymaterial” refers to a rare earth oxide (Ln₂O₃), rare earth disilicates(Ln₂Si₂O₇), rare earth monosilicates (Ln₂SiO₅), a rare earth (Ln)element, a rare earth (Ln) containing aluminosilicate glass, a rareearth (Ln) and alkaline earth containing aluminosilicate glass, a rareearth aluminate (such as rare earth aluminum garnet (Ln₃Al₅O₁₂) andmonoclinic rare earth aluminate (Ln₄Al₂O₉)), phosphorous pentoxide(P₂O₅), a phosphorous containing aluminosilicate glass, a phosphorousand alkaline earth containing aluminosilicate glass, aluminumorthophosphate (AlPO₄), aluminum oxide, and various mixtures thereof. Insome embodiments, secondary materials can help make the coating morecompliant and more likely to stick to the underlying ceramic component.In other embodiments, the secondary materials can contribute to improvedresistance to calcium magnesium aluminum silicate (CMAS), or steam. Inother embodiments, the secondary materials can be more steam recessionresistant than the primary materials.

Each transition layer 16 may have a thickness of from about 0.1 mils toabout 6.0 mils, and may be made and applied to bond coat layer 14 as setforth below. In one embodiment, there may be more than one transitionlayer present. In such instances, each transition layer may comprise thesame or different combination of primary transition materials andsecondary materials. Transition layer 16 may have a porosity level offrom 0% to about 30% by volume of the transition layer, and in anotherembodiment, from about 0.01% to about 30% by volume of the transitionlayer.

As used herein throughout, “rare earth” or “Ln” refers to scandium (Sc),yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium(Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd),terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and lutetium (Lu), while “rare earth oxide” refers toSc₂O₃, Y₂O₃, La₂O₃, CeO₂, Pr₂O₃, Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃,Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and Lu₂O₃. “Alkaline earth”can include magnesium (Mg), calcium (Ca), strontium (Sr), and barium(Ba).

Similarly, outer layer 18 may include from about 85% to about 100% byvolume of the outer layer of a primary outer material and up to about15% by volume of the outer layer of the previously defined secondarymaterial, and in one embodiment from about 85% to about 99% by volume ofthe outer layer of a primary outer material and from about 1% to about15% by volume of the outer layer of the secondary material, though forany EBC, any secondary material present in the outer layer need not bethe same secondary material as is present in the transition layer. Asused herein, “primary outer material” refers to BSAS. Outer layer 18 mayhave a thickness of from about 0.1 mils to about 40 mils, and may besuitable for use in service at temperatures up to about 2500° F. (1371°C.). Outer layer 16 may have a porosity level of from 0% to about 15% byvolume of the outer layer, and in another embodiment, from about 0.01%to about 15% by volume of the outer layer. In this way, the outer layercan help seal out high temperature steam, which can rapidly oxidize theunderlying bond coat layer and ceramic component. Also, because theouter layer can seal out high temperature steam, it can protect theunderlying ceramic component, or EBC layers, from volatilization sincesuch materials may be prone to conversion to gaseous species in hightemperature steam environments.

The layered EBC can be made and applied in accordance with thedescription herein below.

Bond coat layer 14 may be applied by plasma spray processes, chemicalvapor deposition processes, electron beam physical vapor depositionprocesses, dipping in molten silicon, sputtering processes, and otherconventional application processes known to those skilled in the art.

As previously described, in some embodiments, silica layer 15 can formduring the service life of the EBC. More particularly, oxygen in thesurrounding atmosphere can diffuse through the outer layer andtransition layer(s) of the EBC and react with the silicon of bond coatlayer 14 to form silica layer 15. Alternately, silica layer 15 may beintentionally deposited by chemical vapor deposition, plasma spray,slurry deposition, or other conventional method.

In the present embodiments, the manufacturing and application processesfor transition layer 16 and outer layer 18 may consist of a slurrydeposition cycle including sintering aids to lower the temperatureneeded to densify the layer. The slurry deposition cycle can generallyinclude slurry formation, slurry application, drying, and sintering,with optional masking, leveling, sintering aid infiltration, maskremoval, and organic processing aid burnout steps, as set forth below.Those skilled in the art will understand that slurries of varyingcomposition can be used to make EBC layers of varying composition andthat multiple slurry deposition cycles can be used to build up the totalthickness of a particular layer (i.e., transition layer or outer layer).The average thickness per slurry deposition cycle depends primarily onthe slurry solids loading, sintering aid concentration, and number ofdip, spray, or paint passes.

The slurry composition for each of the transition and outer layers ofthe EBC can generally comprise a mixture including from about 9 wt % toabout 81 wt % water; from about 3 wt % to about 72 wt % primarymaterial; from about 0 wt % to about 6 wt % dispersant; from about 0 wt% to about 7 wt % plasticizer; from about 0 wt % to about 1 wt %surfactant; from about 0 wt % to about 18 wt % slurry sintering aid, andin one embodiment from about 0.1 wt % to about 18 wt % slurry sinteringaid; from about 0 wt % to about 11 wt % secondary additive forcontrolled dispersion; from about 0 wt % to about 0.5 wt % thickener;and from about 0 wt % to about 15 wt % latex binder, as set forth hereinbelow.

More specifically, as used herein, “primary material” refers to any ofthe previously defined primary transition materials or primary outermaterials, depending on which layer is being formed. The primarymaterial may comprise a powder having a particle size distribution ofD50 of 0.2-2 microns and D95 of 10-30 microns. Such fine particle sizedistribution can sinter to a dense layer in a reasonable amount of time(i.e. less than about 24 hours). In an alternate embodiment, thedistribution can be bimodal, wherein from about 0.1 vol. % to about 40vol % of the primary material powder particles have a larger sizedescribed by the following distribution: D50 of 10-30 microns and D95 upto 100 microns. Having up to about 40% larger sized particles in abimodal distribution will allows for the formation of thicker layerswith each coating pass without rendering “sluggish” (i.e. greater thanabout 24 hours) sintering behavior.

As used herein, “dispersant” refers to compositions selected from thegroup consisting of polyacrylic acid, polyacrylic acid-polyethyleneoxide copolymers, polymethacrylic acid, polyethylenimine, ammoniumpolyacrylate, ammonium polymethacrylate, sulfonated naphthaleneformaldehyde condensate, polyvinyl sulfonic acid, and combinationsthereof.

As used herein, “plasticizer” refers to compositions selected from thegroup consisting of ethylene glycol, diethylene glycol, triethyleneglycol, tetraethylene glycol glycerol, glycerin, polyethylene glycol,diethylene glycol monobutyl ether, and combinations thereof.

As used herein, “surfactant” refers to compositions selected from thegroup consisting of fluorocarbons, dimethylsilicones, and acetylenicglycol chemistries (e.g. commercial surfactants in the Surfynol® series(Air Products and Chemicals, Inc.)).

As used herein, “slurry sintering aid” refers to sintering aidcompositions suitable for inclusion in the slurry that can be selectedfrom the group consisting of a rare earth nitrate, rare earth acetate,rare earth chloride, rare earth oxide, ammonium phosphate, phosphoricacid, polyvinyl phosphonic acid, and combinations thereof.

As used herein, “secondary additive for controlled dispersion” refers tocompositions selected from the group consisting of citric acid, glycine,dextrose, sucrose, mannose, tartaric acid, oxalic acid, and combinationsthereof.

As used herein, “thickener” refers to compositions selected from thegroup consisting of xanthan gum, polyethylene oxide, guar gum,methylcellulose and other soluble fiber, polyacrylic acid,polyvinylpyrolidone, and combinations thereof.

As used herein, “latex binder” refers to compositions selected from thegroup consisting of polystyrene, polyvinyl alcohol, polyvinyl butyrol,styrene-butadiene copolymer, polyacrylic acid, polyacrylates, acrylicpolymers, polymethyl methacrylate/polybutyl acrylate, polyvinyl acetate,polyvinyl malate, and natural latex rubber. Some examples of latexbinders can include Rhoplex® HA-8, Rhoplex® HA-12, Pavecryl® 2500 (Rohmand Haas).

Also, as used herein, “organic processing aids” refers to anydispersants, plasticizers, secondary additives for controlleddispersion, thickeners, and latex binders present in the slurry. Theseorganic processing aids are comprised primarily of carbon and otherelements that volatilize during processing such that they are notpresent in the post-sintered coating.

In one embodiment, the slurry can be formed by combining any water,dispersant, primary material, surfactant, and plasticizer desired withmixing media in a container for from about 3 hours to about 15 hours.The mixture can be mixed using conventional techniques known to thoseskilled in the art such as shaking with up to about a 0.039 inch (1 mm)to 0.25 inch (6.35 mm) diameter alumina or zirconia mixing media, ballmilling using about a 0.25 inch to about a 1 inch (about 6.35 mm toabout 25.4 mm) diameter alumina or zirconia mixing media, attritormilling using about a 1 mm to about a 5 mm diameter zirconia-basedmixing media, planetary ball milling using from about a 1 mm to about a5 mm diameter zirconia-based media, or mechanical mixing or stirringwith simultaneous application of ultrasonic energy. The mixing media orultrasonic energy can break apart any agglomerated ceramic particles inthe slurry. Any mixing media present may subsequently be removed bystraining, for example.

Thickener may be added to the slurry if desired and the resultingmixture may be agitated by such methods as mechanical stirring, rolling,blending, shaking, and other like methods. Once the thickener is fullydissolved, generally after about 5 minutes to about 60 minutes, anysecondary additive for controlled dispersion can be added if desired,and the resulting slurry may again be mixed using any of the abovelisted methods until the secondary additive dissolves, which again canbe after from about 5 minutes to about 60 minutes. The addition ofsintering aid may follow if desired, along with mixing using thepreviously described methods until the sintering aid dissolves, whichcan take from about 5 minutes to about 60 minutes. The latex binder maythen be added if desired, and the slurry may be mixed by slow rolling,slow mechanical mixing, or other like methods to avoid trapping airbubbles in the slurry. This light mixing can be continued indefinitely,or alternately, once mixed, the slurry can be set aside until needed forapplication. In one embodiment, the slurry may be refreshed by addingadditional water to account for that which has evaporated duringprocessing.

Those skilled in the art will understand that the previous embodiment isone method for making the slurry compositions described herein, and thatother methods are also acceptable, as set forth in the Examples below.

If desired, masking can be applied to the ceramic component before theslurry is applied to prevent coating specific areas of the component.Masking may be accomplished using conventional techniques known to thoseskilled in the art including, but not limited to, tapes, tooling, andpaint-on adhesives.

Once all desired masking of the ceramic component is complete, theslurry can be applied to produce a coated component. The slurry can beapplied directly to the ceramic component using any conventional slurrydeposition method known to those skilled in the art, including but notlimited to, dipping the component into a slurry bath, or painting,rolling, stamping, spraying, or pouring the slurry onto the component.In one embodiment, slurry application can be carried out in a humidenvironment to help prevent water evaporation that could change theslurry rheology, for example, during coating deposition onto a largebatch of parts. In one embodiment, the environment can comprise greaterthan 50% relative humidity, in another embodiment greater than 70%relative humidity, and in yet another embodiment greater than 95%relative humidity, all at or near room temperature (about 20° C. toabout 30° C.). Slurry application can be carried out manually or it maybe automated.

Once the slurry has been applied to the ceramic component, and while theslurry is still wet, it may be leveled to remove excess slurry material.Leveling may be carried out using conventional techniques such as, butnot limited to, spinning, rotating, slinging the component, drippingwith or without applied vibration, or using a doctor blade, to removeexcess slurry material. Similar to slurry application, leveling can beconducted manually or it may be automated, and it can be carried out ina humid environment because if the slurry dries too quickly, it can leadto defects in the coating during leveling.

Next, the coated component can be dried. Drying may be carried out inambient or controlled temperature and humidity conditions. In oneembodiment, controlled temperature and humidity can be utilized to helpmaintain the integrity of the applied slurry coating. More particularly,in one embodiment, drying may be carried out at temperatures of fromabout 5° C. to about 100° C., and in another embodiment, from about 20°C. to about 30° C., and in an environment comprising from about 10%relative humidity to about 95% relative humidity, in one embodiment fromabout 50% relative humidity to about 90% relative humidity, and in yetanother embodiment from about 70% relative humidity to about 80%relative humidity.

After drying, any masking present may be removed by peeling off tapesand adhesives, pyrolysis of tapes and adhesives, or by removingmulti-use tooling. Any rough edges remaining after masking removal maybe scraped or cut away using conventional means.

Next, burnout of the organic processing aids may be carried out byplacing the dried component in an elevated temperature environment sothat any bound water can be evaporated and the organic processing aidscan be pyrolyzed. In one embodiment, burnout of the organic processingaids may be accomplished by heating the dried component at a rate offrom about 1° C./min to about 15° C./min to a temperature of from about400° C. to about 1000° C. and holding the component at this temperaturefor from about 0 to about 2 hours. In another embodiment, the coatedcomponent may be heated at a rate of from about 2° C./min to about 6°C./min to a temperature of from about 600° C. to about 800° C. andholding the component at this temperature for from about 0 to about 2hours. In another embodiment, the hold time can be eliminated by slowlyramping up to the target temperature without holding, followed byramping up or down to another temperature at a different rate. Inanother embodiment, binder burnout can occur rapidly by placing thecoated component into a furnace heated to a temperature of from about400° C. to about 1400° C.

The dried component may then be sintered to produce a componentcomprising an environmental barrier coating. Sintering can serve tosimultaneously densify and impart strength to the coating. Additionally,in the case of the outer layer of the EBC, sintering can impart ahermetic seal against high temperature steam present in the engineenvironment. Sintering can be carried out using a conventional furnace,or by using such methods as microwave sintering, laser sintering,infrared sintering, and the like.

Sintering can be accomplished by heating the dried component at a rateof from about 1° C./min to about 15° C./min to a temperature of fromabout 1100° C. to about 1700° C. and holding the component at thattemperature for from about 0 to about 24 hours. In another embodiment,sintering can be accomplished by heating the coated component at a rateof from about 5° C./min to about 15° C./min to a temperature of fromabout 1300° C. to about 1375° C. and holding the component at thattemperature for from about 0 to about 24 hours.

Alternately, in another embodiment, binder burnout and sintering can becarried out in a single process by heating at a rate of about 1°C./minute to about 15° C./minute to a temperature of from about 400° C.to about 1000° C. and holding at this temperature for from about 0 toabout 2 hours. The component can then be heated at a rate of from about1° C./minute to about 15° C./minute from the binder burnout temperatureto from about 1100° C. to about 1700° C. and holding at his temperaturefor from about 0 to about 24 hours, as set forth in the Examples below.

In an alternate embodiment, all layers of the EBC can be applied, one ontop of the other, before masking removal, organic processing aidburnout, and sintering are carried out. Those skilled in the art willunderstand that after application of each layer, the layer should be atleast partially dried before application of the subsequent layer.

In another embodiment, the sintering aid does not need to be addeddirectly to every layer of the slurry to achieve the desired result. Forexample, in one embodiment, a slurry comprising at least the primarytransition material of mullite, or a mullite/BSAS mixture, with nosintering aid may be applied to the ceramic component as a transitionlayer, dried, and sintered in either a vacuum, an inert atmosphere, or areducing atmosphere (argon, argon 4% hydrogen, nitrogen, etc.). Next, aBSAS (primary outer material) slurry containing a sintering aid can beapplied as the outer layer. Upon drying and sintering the outer layer,the sintering aid from the BSAS outer layer can diffuse into thetransition layer to form secondary materials. Alternately, thetransition layer slurry can comprise the sintering aid while the outerlayer slurry does not. In this instance, during sintering the sinteringaid can diffuse from the transition layer to the outer layer. In anotherembodiment, a primary material slurry with no sintering aid can bedensified by applying the layer, allowing it to dry, and then backinfiltrating a sol-gel solution comprised of a water soluble or solventsoluble sintering aid prior to heat treatment as explained below.

Infiltration of a sol-gel solution may be useful as some of the rareearth-containing and phosphorous-containing sintering aids can bedifficult to mix into the previously described water-based slurry.Infiltration may also allow for the densification of a thicker layer ofEBC material at one time. Moreover, infiltration is also a way to addmore sintering aid after sintering if the coating isn't as dense asdesired. The sol-gel solution used for infiltration may be an aqueoussolution of a “water soluble sintering aid” selected from the groupconsisting of a rare earth nitrate, a rare earth acetate, a rare earthchloride, phosphoric acid, ammonium phosphate, ammonium phosphatedibasic, ammonium phosphate monobasic, or polyvinyl phosphoric acid. Inanother embodiment, the sol-gel solution may comprise a solution oforganic solvent and a “solvent soluble sintering aid.” As used herein,“solvent soluble sintering aid” refers to a “solvent soluble rare earthsource,” or a “solvent soluble phosphorous source,” as defined hereinbelow.

As used herein, “organic solvent” refers to solvents including, but notlimited to, methyl alcohol, ethyl alcohol, isopropanol, butyl alcohol,pentanol, hexanol, heptanol, octanol, glycerol, glycerin, polyethyleneglycol, ethylene glycol, acetone, toluene, xylene, heptane, methylisobutyl ketone (MIBK), ethylbenzene, propylbenzene, heptane, octane,nonane, decane, and mixtures thereof.

As used herein, “solvent soluble rare earth source” refers to, but isnot limited to, a rare earth acetate, a rare earth oxalate, a rare earthisopropoxide, a rare earth methoxyethoxide, rare earth2,2,6,6-tetramethyl-3,5-heptanedionate, rare earth 2,4-pentanedionate,rare earth acetylacetonate, and mixtures thereof.

As used herein “solvent soluble phosphate source” refers to, but is notlimited to, polyvinyl phosphoric acid; tributyl phosphate; trimethylphosphate; triethyl phosphate; tripropyl phosphate; urea phosphate salt;mono-methyl phosphate bis(cyclohexylammonium) salt; dimethyl phosphate;2-aminoethyl dihydrogen phosphate; phospho(enol)pyruvic acidcyclohexylammonium salt; trimethyl phosphate; diethyl phosphate;anilinium hypophosphite;4-ethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane; phytic acid; diethylallyl phosphate; spermidine phosphate salt hexahydrate; dibutylphosphate; tetraethyl pyrophosphate; triisopropyl phosphate; 1-naphthylphosphate; bis(4-nitrophenyl)phosphate; spermine diphosphate salt;diphenyl phosphate; 2,4-diamino-6,7-diisopropylpteridine phosphate salt;cyclohexylammonium phosphate dibasic; tributyl phosphate; tridecylphosphate; dibenzyl phosphate; bis(2-ethylhexyl)phosphate; triphenylphosphate; triphenyl phosphate; tris(2-butoxyethyl)phosphate; tricresylphosphate; hydroxypyruvic acid dimethyl ketal phosphatetri(cyclohexylammonium) salt; tris(2-ethylhexyl)phosphate; tetrabenzylpyrophosphate;{3,9-Bis(2,4-dicumylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane};or mixtures thereof.

As used herein, “sintering aid(s)” refers to any of a “slurry sinteringaid,” a “water soluble sintering aid,” or “a solvent soluble sinteringaid,” as defined previously. Without intending to be limited by theory,the inclusion of sintering aids to the EBC embodiments herein canincrease the rate of diffusion of the primary material such that surfacearea reduction (i.e. high surface area particles consolidating to form adense coating) can occur at lower temperatures than it would have absentthe sintering aid. As previously described, sintering at lowertemperatures (i.e. about 1375° C. or below) can not only result in ahighly dense (i.e. greater than about 70% for the transition layer andgreater than about 85% for the outer layer) coating that can be lesssusceptible to the penetration of hot steam from the engine environment,but can also help prevent the degradation of the mechanical propertiesof the underlying component that could result from prolonged exposure tohigher temperatures.

Sintering aids can act in a variety of ways depending on the amount ofsintering aid included in the EBC and the time at which the coating isexposed to sintering temperatures. For example, in one embodiment, thesintering aid can dissolve completely into the primary material (i.e.primary transition materials or primary outer materials). In anotherembodiment, if the amount of sintering aid that is soluble in theprimary material is exceeded, the remaining insoluble portion ofsintering aid can react with the primary material to form the secondarymaterial (i.e. secondary transition material or secondary outermaterial). In another embodiment, primary material and secondarymaterial can be present as described previously, along with residualsintering aid. In these latter two embodiments, when the secondarymaterial is highly volatile in high temperature steam, such as aluminumphosphate, as long as the total volume of secondary material, plusporosity (plus residual sintering aid when present) in the outer layerof the EBC remains about 15% or less, the hermetic seal can bemaintained. Alternately, in these latter two embodiments, when thesecondary material is highly resistant to volatilization in hightemperature steam, such as when the secondary material comprises a rareearth disilicate, rare earth monosilicate, or rare earth aluminate, thenonly the porosity in the outer layer of the EBC need remain about 15% orless to maintain the hermetic seal.

It should be noted that at low levels of sintering aid, the densifiedcoating layer might not initially include any detectable secondarymaterials. In some embodiments, the secondary materials may never becomedetectable. In other embodiments, however, after hours of exposure tohigh temperature steam in the engine environment, the secondarymaterials can become detectable using techniques such as x-raydiffraction, electron microscopy, electron dispersive spectroscopy, andthe like.

EBC embodiments described herein can offer a variety of benefits overcurrent EBCs and manufacturing processes thereof. Specifically, aspreviously described, the inclusion of a sintering aid in the EBCembodiments herein can permit sintering at lower temperatures (i.e.about 1357° C. or below). This can result in a highly dense (i.e.greater than about 70% for the transition layer and greater than about85% for the outer layer) coating that can be less susceptible to thepenetration of hot steam from the engine environment, and can also helpprevent the degradation of the mechanical properties of the underlyingcomponent that could result from prolonged exposure to highertemperatures. Also, the embodiments set forth herein can be made at lessexpense than current EBCs due to the use of the slurry depositionprocess, which is made possible by the incorporation of sintering aidsinto the various layers. Moreover, the present embodiments can providefor EBCs having a more uniform thickness than conventional techniques,such as plasma spraying, even when applying thin layers (<2 mils). Theslurry deposition process can allow for the application of the EBCs tointernal component passages as well as the ability to produce smoothsurface finishes without an additional polishing step.

There can be occasions when the EBC develops small and/or narrow defects(e.g. about 10 microns to about 5 mm in diameter; or about 10 microns toabout 1 mm in width) that need to be repaired. The following repairprocesses are applicable to the EBCs described herein and may be carriedout after sintering of an individual EBC layer, or after sintering theentire applied EBC, as explained herein below.

In one embodiment, repairs may include remedying defects in one or moreindividual layers as the EBC is being applied using the methodsdescribed herein. In this embodiment, the repair can be carried outafter sintering a given layer by applying a repair slurry comprising thesame slurry materials used to make the layer having the defects. Forexample, if the transition layer develops a defect after sintering, thedefect could be repaired using a “transition layer repair slurry” thatcomprises the same transition layer slurry materials used in theoriginal application of the transition layer. In one embodiment, therepair slurry can comprise a higher solids loading of primary materialceramic particles than the original slurry layer as this can reduceshrinkage on drying and sintering of the repaired portion of thecoating. In particular, the solids loading of primary material ceramicparticles in the repair slurry can be greater than about 30% to about55% by volume (as opposed to greater than about 10% by volume in oneembodiment of the original slurry, and from about 10% to about 55% byvolume in another embodiment of the original slurry used to make thelayer). The repair slurry may be applied using any conventional methodincluding those described previously, and the resulting “repair(ed)coating” may then be processed as described previously herein beforeapplication of any subsequent layer of the EBC.

In an alternate embodiment, repairs may include fixing defects afterapplication and sintering of the entire EBC. In this embodiment, therepair may be carried out on the EBC having defects using an outer layerrepair slurry comprising the same materials present in the previouslydefined outer layer slurry (i.e. primary outer material, a sinteringaid, and optionally secondary material). This particular repair slurrycan seep into any defects present in the EBC and provide a hermetic sealto the repaired EBC coating after sintering. Again, the solids loadingof the outer layer repair slurry may comprise upwards of about 30% to55% by volume.

Additionally, repair processes may be used to reduce surface roughnessof a plasma sprayed EBC having any composition. Specifically, if thesurface roughness of a plasma sprayed EBC is unacceptable the coatingcan be smoothed over by applying the previously described outer layerrepair slurry. When applied over the plasma sprayed EBC, the outer layerrepair slurry can fill in any gaps, grooves, or uneven portions of theplasma sprayed coating and reduce the surface roughness to an acceptabledegree. More specifically, depending on the thickness of the outer layerrepair slurry, surface roughness of the plasma sprayed EBC can bereduced from greater than 200 micro inch Ra, to between 40 micro inch Raand 200 micro inch Ra in one embodiment, and from between 40 micro inchRa to 150 micro inch Ra in another embodiment. In one embodiment, theouter layer repair slurry comprises a thickness of at least about 0.5mils, and in another embodiment from about 0.5 mils to about 3 mils. Theprimary outer material solids loading for surface roughness improvementcan be greater than about 10% to about 55% by volume in one embodiment,and from about 30% to about 55% by volume in another embodiment. Theapplied outer layer repair slurry can then be processed as describedpreviously to produce a repaired EBC having an acceptable surfaceroughness.

Such repair processes can provide the ability to repair localizeddefects, at varying points during the application or life of thecoating, as opposed to stripping off and reapplying the entire coating.This, in turn, can result in a savings of time, labor, and materials.

EXAMPLES Example 1

A silicon bond coat was applied to a SiC—SiC CMC using a conventionalair plasma spray process. Next, a primary transition material slurry wasmade by first mixing mullite, BSAS, water, polyacrylic acid, Surfynol502®, and glycerin in a plastic container, along with enough 0.25 inch(6.35 mm) diameter, cylindrical alumina media to line the bottom ofcontainer. This mixture was placed on a roller mill for 15 hours. Aftertaking the container off of the roller mill, the alumina media wasremoved. Xanthan gum was then added and the mixture was shaken for 15minutes using a paint shaker. Finally, Rhoplex® HA8 emulsion was addedand the container was placed back onto the roller mill for 1 hour(without media).

The resulting primary transition material slurry (Slurry A) consisted of40.68% mullite, 10.17% BSAS, 2.54% polyacrylic acid, 0.14% Surfynol502®, 0.23% xanthan gum, 6.71% Rhoplex® HA8 emulsion, 4.92% glycerin,and the balance water (all percents by weight). No sintering aid wasadded to this slurry. The silicon-coated ceramic component was dippedinto Slurry A, dried in ambient conditions, and heat-treated at 3°C./minute to 1000° C. to burn out the binder. Then, the component wassintered by heating the component at 5° C./minute from 1000° C. to 1344°C. and holding for 5 hours to form a transition layer comprising amullite/BSAS mixture (80 vol % mullite and 20 vol % BSAS). The entireheat treatment was carried out in a vacuum. The heating environmentresulted in the mullite/BSAS transition layer having a porosity of lessthan 10% by volume.

Next, a primary outer material slurry was made by first mixing BSAS,water, polyacrylic acid, Surfynol 502®, and glycerin in a plasticcontainer, along with enough 0.25 inch (6.35 mm) diameter, cylindricalalumina media to line the bottom of container. This mixture was placedon a roller mill for 15 hours. After taking the container off of theroller mill, the alumina media was removed. Next, glycine, xanthan gum,and yttrium nitrate hexahydrate were added and the mixture was shakenfor 15 minutes using a paint shaker. Finally, Rhoplex® HA8 emulsion wasadded and the container was placed back onto the roller mill for 1 hour(without media).

The resulting primary outer material slurry (Slurry B) consisted of49.49% BSAS, 1.84% yttrium nitrate hexahydrate (slurry sintering aid),2.47% polyacrylic acid, 4.91% glycine, 0.13% Surfynol® 502, 6.36%Rhoplex® HA8 emulsion, 4.34% glycerin, and the balance water (allpercents by weight). The SiC—SiC CMC with bond coat and transition layerwas dipped into Slurry B, dried in ambient conditions, and heat-treatedat 3° C./minute to 1000° C. to burn out the binder. Then, the componentwas sintered by heating the component at 5° C./minute from 1000° C. to1344° C. and holding for 5 hours to form a densified outer layer. Theentire heat treatment was carried out in ambient conditions. The SlurryB coating process and heat treatment were then repeated one time toincrease the thickness of the outer layer. The outer layer had aporosity of less than 10% by volume due to the yttrium nitrate slurrysintering aid in Slurry B.

FIG. 2. shows a SEM micrograph of this coating microstructure with theair plasma spray silicon bond coat (100), transition layer (102), andouter layer (104). While the outer layer and transition layer appearedto consist mainly of the primary materials (BSAS and mullite/BSASmixture, respectively), x-ray diffraction suggested that there was asecondary material present in the EBC, i.e., yttrium disilicate.Combined x-ray diffraction and EDS analysis suggested that there weresmall amounts of rare earth aluminosilicate glass and alkaline earthaluminosilicate glass secondary materials in the outer layer as well.

Example 2

A silicon bond coat was applied to a SiC—SiC CMC by a conventional airplasma spray process. Next, a primary transition material slurry wasmade by first mixing mullite, BSAS, water, polyacrylic acid, Surfynol502®, and glycerin in a plastic container, along with enough 0.25 inch(6.35 mm) diameter, cylindrical alumina media to line the bottom ofcontainer. This mixture was placed on a roller mill for 15 hours. Aftertaking the container off of the roller mill, the alumina media wasremoved. Xanthan gum and ammonium phosphate monobasic were then addedand the mixture was shaken for 15 minutes using a paint shaker. Finally,Rhoplex® HA8 emulsion was added and the container was placed back ontothe roller mill for 1 hour (without media).

The resulting primary transition material slurry (Slurry C) consisted of34.07% mullite, 8.52% BSAS, 4.51% ammonium phosphate monobasic (slurrysintering aid), 2.13% polyacrylic acid, 0.16% Surfynol® 502, 0.29%xanthan gum, 5.62% Rhoplex® HA8 emulsion, 2.56% glycerin, and thebalance water (all percents by weight). The silicon-coated ceramiccomponent was dipped into Slurry C, dried in ambient conditions, andheat-treated at 3° C./minute to 1000° C. to burn out the binder. Then,the component was sintered by heating the component at 5° C./minute from1000° C. to 1344° C. and holding for 5 hours to form a densifiedtransition layer comprising a mullite/BSAS mixture (80 vol % mullite and20 vol % BSAS). The entire heat treatment was carried out in ambientconditions. The densified transition layer had a porosity of less than10% by volume due to the ammonium phosphate monobasic slurry sinteringaid in Slurry C.

Next, a primary outer material slurry was made by first mixing BSAS,water, polyacrylic acid, Surfynol 502®, and glycerin in a plasticcontainer, along with enough 0.25 inch (6.35 mm) diameter, cylindricalalumina media to line the bottom of the container. This mixture wasplaced on a roller mill for 15 hours. After taking the container off ofthe roller mill, the alumina media was removed. Xanthan gum and ammoniumphosphate monobasic were then added and the mixture was shaken for 15minutes using a paint shaker. Finally, a Rhoplex® HA8 emulsion was addedand the container was placed back onto the roller mill for 1 hour(without media).

The resulting primary outer material slurry (Slurry D) consisted of50.55% BSAS, 1.60% ammonium phosphate monobasic (slurry sintering aid),2.53% polyacrylic acid, 0.13% Surfynol® 502, 0.25% xanthan gum, 6.50%Rhoplex® HA8 emulsion, 5.91% glycerin, and the balance water. TheSiC—SiC CMC with bond coat and transition layer was dipped into SlurryD, dried in ambient conditions, and heat-treated at 3° C./minute to1000° C. to burn out the binder. Then, the component was sintered byheating the component at 5° C./minute from 1000° C. to 1344° C. andholding for 5 hours to form a densified outer layer. The entire heattreatment was carried out in ambient conditions. The Slurry D coatingprocess and heat treatment were then repeated to increase the thicknessof the outer layer. The densified outer layer had a porosity of lessthan 10% by volume due to the ammonium phosphate monobasic slurrysintering aid in Slurry D.

FIG. 3 shows a SEM micrograph of a CMC (105) having this coatingmicrostructure with the air plasma spray silicon bond coat (106),transition layer (108), and outer layer (110). While the outer layer andtransition layer appeared to consist of the primary materials (BSAS andmullite/BSAS mixture, respectively), x-ray diffraction suggested thatthere were secondary materials present, including aluminum oxide, andyttrium aluminate (Y₃Al₅O₁₂).

Example 3

A silicon bond coat was applied to a SiC—SiC CMC using a conventionalair plasma spray process. Next, a primary transition material slurry wasmade by first mixing mullite, BSAS, water, polyacrylic acid, Surfynol502®, and glycerin in a plastic container, along with enough 0.25 inch(6.35 mm) diameter, cylindrical alumina media to line the bottom ofcontainer. This mixture was placed on a roller mill for 15 hours. Aftertaking the container off of the roller mill, the alumina media wasremoved. Glycine, xanthan gum, and yttrium nitrate hexahydrate were thenadded and the mixture was shaken for 15 minutes using a paint shaker.Finally, a Rhoplex® HA8 emulsion was added and the container was placedback onto the roller mill for 1 hour (without media).

The resulting primary transition material slurry (Slurry E) consisted of32.23% mullite, 8.06% BSAS, 8.04% yttrium nitrate hexahydrate (slurrysintering aid), 2.01% polyacrylic acid, 5.55% glycine, 0.15% Surfynol®502, 0.10% xanthan gum, 5.31% Rhoplex® HA8 emulsion, 3.63% glycerin, andthe balance water (all percents by weight). The silicon-coated ceramiccomponent was dipped into slurry E, dried in ambient conditions, andheat-treated at 3° C./minute to 1000° C. to burn out the binder. Then,the component was sintered by heating the component at 5° C./minute from1000° C. to 1344° C. and holding for 5 hours to form a densifiedtransition layer comprising a mullite/BSAS mixture (80 vol % mullite and20 vol % BSAS). The entire heat treatment was carried out in ambientconditions. The yttrium nitrate hexahydrate slurry sintering aidpromoted densification of the transition layer to a porosity of lessthan 10% by volume.

Next, a primary outer material slurry was made by first mixing BSAS,water, polyacrylic acid, Surfynol 502®, and glycerin in a plasticcontainer, along with enough 0.25 inch (6.35 mm) diameter, cylindricalalumina media to line the bottom of container. This mixture was placedon a roller mill for 15 hours. After taking the container off of theroller mill, the alumina media was removed. Xanthan gum was added andthe mixture was shaken for 15 minutes using a paint shaker. Finally,Rhoplex® HA8 emulsion was added and the container was placed back ontothe roller mill for 1 hour (without media).

The resulting primary outer material slurry (Slurry F) consisted of57.07% BSAS, 2.85% polyacrylic acid, 0.12% Surfynol® 502, 0.19% xanthangum, 7.34% Rhoplex® HA8 emulsion, 5.30% glycerin, and the balance water(all percents by weight). No sintering aid was added to Slurry F. TheSiC—SiC CMC with bond coat and transition layer was dipped into SlurryF, dried in ambient conditions, and heat-treated at 3° C./minute to1000° C. to burn out the binder. Then, the component was sintered byheating the component at 5° C./minute from 1000° C. to 1344° C. andholding for 5 hours to form a densified outer layer. The entire heattreatment was carried out in ambient conditions. In this case, residualsintering aid from the transition layer diffused into the outer layer topromote densification of the outer layer to a porosity of less than 10%by volume.

FIG. 4 shows a SEM micrograph of a CMC (111) having this coatingmicrostructure with the air plasma spray silicon bond coat (112),transition layer (114), and outer layer (116). While the outer layer andtransition layer appear to consist mainly of the primary materials (BSASand mullite/BSAS mixture, respectively), x-ray diffraction suggestedthat there was a secondary material present, i.e., yttrium disilicate.Additionally, some glassy material had also formed in the transitionlayer due to the reaction between mullite and BSAS.

Example 4

A silicon bond coat was applied to a SiC—SiC CMC using a conventionalair plasma spray process. Next, a primary transition material slurry wasmade by mixing mullite, BSAS, water, polyacrylic acid, Surfynol 502®,and glycerin in a plastic container, along with enough 0.25 inch (6.35mm) diameter, cylindrical alumina media to line the bottom of container.This mixture was placed on a roller mill for 15 hours. After taking thecontainer off of the roller mill, the alumina media was removed. Xanthangum was then added and the mixture was shaken for 15 minutes using apaint shaker. Finally, Rhoplex® HA8 emulsion was added and the containerwas placed back onto the roller mill for 1 hour (without media).

The resulting primary transition material slurry (Slurry G) consisted of40.68% mullite, 10.17% BSAS, 2.54% polyacrylic acid, 0.14% Surfynol®502, 0.23% xanthan gum, 6.71% Rhoplex® HA8 emulsion, 4.92% glycerin, andthe balance water (all percents by weight). No sintering aid was addedto slurry G. The silicon-coated ceramic component was dipped into slurryG and dried in ambient conditions. Subsequently, the coated componentwas immersed in a sintering aid solution of 8 wt % yttriummethoxyethoxide (solvent soluble sintering aid) and 92 wt % ethylalcohol (organic solvent) so that the sintering aid solution could flowinto the pore space within the unfired coating. The ethyl alcoholevaporated under ambient conditions and the coated component washeat-treated at 3° C./minute to 1000° C. to burn out the binder. Then,the component was sintered by heating the component at 5° C./minute from1000° C. to 1344° C. and holding for 5 hours to form a densifiedtransition layer comprising a mullite/BSAS mixture (80 vol % mullite and20 vol % BSAS) and having a porosity of less than 10% by volume.

Next a primary outer material slurry was made by mixing BSAS, water,polyacrylic acid, Surfynol 502®, and glycerin in a plastic container,along with enough 0.25 inch (6.35 mm) diameter, cylindrical aluminamedia to line the bottom of container. This mixture was placed on aroller mill for 15 hours. After taking the container off of the rollermill, the alumina media was removed. Xanthan gum was then added and themixture was shaken for 15 minutes using a paint shaker. Finally,Rhoplex® HA8 emulsion was added and the container was placed back ontothe roller mill for 1 hour (without media).

The resulting primary outer material slurry (Slurry H) consisted of51.48% BSAS, 2.57% polyacrylic acid, 0.14% Surfynol® 502, 0.23% xanthangum, 6.62% Rhoplex® HA8 emulsion, 4.86% glycerin, and the balance water(all percents by weight). No sintering aid was added to Slurry H. TheSiC—SiC CMC with bond coat and transition layer was dipped into Slurry Hand dried in ambient conditions. Subsequently, the coated component wasimmersed in a sintering aid solution of 8 wt % yttrium methoxyethoxideand 92 wt % ethyl alcohol so that the sintering aid solution could flowinto the pore space within the unfired coating. After drying out theethyl alcohol as described previously, the sample was heat-treated at 3°C./minute to 1000° C. to burn out the binder. Then, the component wassintered by heating the component at 5° C./minute from 1000° C. to 1344°C. and holding for 5 hours to form a densified outer layer having aporosity of less than 10% by volume. The entire heat treatment wascarried out in ambient conditions.

FIG. 5 shows a SEM micrograph of a CMC (117) having this coatingmicrostructure with the air plasma spray silicon bond coat (118),mullite/BSAS transition layer (120), and the BSAS outer layer (122).While the BSAS outer layer and mullite/BSAS transition layer appear toconsist mainly of the primary materials, x-ray diffraction suggestedthat there was a secondary material present, i.e., yttrium disilicate.Combined x-ray diffraction and EDS analysis suggested that there weresmall amounts of rare earth aluminosilicate glass and alkaline earthaluminosilicate glass secondary materials in the outer layer as well.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

What is claimed is:
 1. An environmental barrier coating comprising: atleast one transition layer made from a transition layer slurrycomposition comprising: water; a primary transition material; and afirst slurry sintering aid in an amount that exceeds solubility of thefirst slurry sintering aid in the primary transition material so thatthe amount of the first slurry sintering includes an insoluble portion;and an outer layer made from an outer layer slurry compositioncomprising: water; a primary outer material; and a second slurrysintering aid in an amount that exceeds solubility of the second slurrysintering aid in the primary outer material so that the amount of thesecond slurry sintering aid includes an insoluble portion, wherein theprimary transition material is selected from the group consisting ofmullite, BSAS, and a mullite/BSAS mixture, and the primary outermaterial comprises BSAS, and wherein the transition layer consists ofthe primary transition material, any of the first slurry sintering aiddissolved therein, and a first secondary material as a reaction productof the primary transition material and the insoluble portion of thefirst slurry sintering aid, the outer layer consists of the primaryouter material, any of the second slurry sintering aid dissolvedtherein, and a second secondary material as a reaction product of theprimary outer material and the insoluble portion of the second slurrysintering aid, the first and second slurry sintering aids increase ratesof diffusion of, respectively, the primary transition material and theprimary outer material such that surface area reduction occurs withinthe transition layer and the outer layer at lower temperatures thanwould occur absent the first and second slurry sintering aids, and thefirst and second secondary materials are chosen from the groupconsisting of rare earth oxides, rare earth disilicates, rare earthmonosilicates, rare earth elements, rare earth containingaluminosilicate glasses, rare earth and alkaline earth containingaluminosilicate glasses, rare earth aluminates, phosphorous pentoxide,phosphorous containing aluminosilicate glasses, phosphorous and alkalineearth containing aluminosilicate glasses, aluminum orthophosphate,aluminum oxide, and combinations thereof.
 2. The coating of claim 1,wherein the transition layer comprises a thickness of about 2.5 to about150 micrometers, and the outer layer comprises a thickness of about 2.5to about 100 micrometers.
 3. The coating of claim 1, wherein the firstand second secondary materials contain at least one of the rare earthdisilicates and rare earth monosilicates.
 4. The coating of claim 1,wherein the environmental barrier coating is on a component formed of aceramic matrix composite material or a monolithic ceramic material andthe component is selected from the group consisting of combustorcomponents, turbine blades, shrouds, nozzles, heat shields, and vanes.5. An environmental barrier coating comprising: a bond coat layercomprising silicon; a silica layer; at least one transition layer madefrom a transition layer slurry composition comprising: from about 9 wt %to about 81 wt % water; from about 3 wt % to about 72 wt % of a primarytransition material; and a first slurry sintering aid in an amount thatexceeds solubility of the first slurry sintering aid in the primarytransition material so that the amount of the first slurry sinteringincludes an insoluble portion; and an outer layer made from a slurrycomposition comprising: from about 9 wt % to about 81 wt % water; fromabout 3 wt % to about 72 wt % of a primary outer material; and a secondslurry sintering aid in an amount that exceeds solubility of the secondslurry sintering aid in the primary outer material so that the amount ofthe second slurry sintering aid includes an insoluble portion; whereinthe transition layer comprises a porosity of from about 0.01% to about30% by volume of the transition layer, and the outer layer comprises aporosity of from about 0.01% to about 15% by volume of the outer layer,and wherein the first and second slurry sintering aids are selected fromthe group consisting of rare earth nitrate, rare earth acetate, rareearth chloride, rare earth oxide, ammonium phosphate, phosphoric acid,polyvinyl phosphonic acid, and combinations thereof, and wherein theprimary transition material is selected from the group consisting ofmullite, BSAS, and a mullite/BSAS mixture, and the primary outermaterial comprises BSAS, and wherein the transition layer consists ofthe primary transition material, any of the first slurry sintering aiddissolved therein, and a first secondary material as a reaction productof the primary transition material and the insoluble portion of thefirst slurry sintering aid, the outer layer consists of the primaryouter material, any of the second slurry sintering aid dissolvedtherein, and a second secondary material as a reaction product of theprimary outer material and the insoluble portion of the second slurrysintering aid, the first and second slurry sintering aids increase ratesof diffusion of, respectively, the primary transition material and theprimary outer material such that surface area reduction occurs withinthe transition layer and the outer layer at lower temperatures thanwould occur absent the first and second slurry sintering aids, and thefirst and second secondary materials are chosen from the groupconsisting of rare earth disilicates, rare earth monosilicates, rareearth aluminates, and combinations thereof.
 6. The coating of claim 5wherein the first and second secondary materials contain at least one ofthe rare earth disilicates and rare earth monosilicates.
 7. The coatingof claim 5, wherein the transition layer comprises a thickness of about2.5 to about 150 micrometers, and the outer layer comprises a thicknessof about 2.5 to about 100 micrometers.
 8. The coating of claim 5,wherein the environmental barrier coating is on a component formed of aceramic matrix composite material or a monolithic ceramic material andthe component is selected from the group consisting of combustorcomponents, turbine blades, shrouds, nozzles, heat shields, and vanes.